The direction in which the compass needle points aboard ship is the resultant of the effect of the earth's magnetic field and the magnetic field of the ship, and both fields are subject to continual and complicated variations from year to year, from day to day, and even from hour to hour!
The elements of the earth's magnetic field are determined for any one epoch by long-continued magnetic surveys carried on to a greater or less extent by the various nations of the world, and the results are published in the form of magnetic charts for land and sea, showing the values of the three magnetic elements, declination of the needle, dip or inclination, and horizontal intensity of the earth's field for a definite period. So rapid are even the long-period changes in the earth's magnetic field that a magnetic chart can be relied upon for only a very few years and fresh data for the construction of these charts that are so valuable to navigators and explorers must be gathered continually.
The Department of Terrestrial Magnetism of the Carnegie Institute of Washington is engaged in continual magnetic surveys of the earth by land and sea that are of the highest value not only to navigators but also to scientists interested in solving the great and mysterious problem of the underlying causes of the earth's magnetism.
To give an idea of the extent and scope of the work of this department it may be mentioned that its non-magnetic ship Carnegie made in the period 1909-1918 a total run of 189,176 nautical miles, nearly nine times the earth's circumference, with an average day's run of 119 nautical miles. Magnetic observations were made practically every day at a distance of 100 to 150 miles apart. In this nine-year period five cruises were made. On her first cruise the Carnegie sailed from St. John's, Newfoundland, to Falmouth, England, over practically the same course followed by the famous astronomer, Halley, in the Paramour Pink two centuries earlier to determine the variation of the compass. In her fourth voyage the Carnegie circumnavigated the world in sub-antarctic regions in 118 days—a record time. She has traversed all oceans from 80° North to the parallel of 60° South and has crossed and recrossed her own path and the path of her predecessor, the Galilee, many times, thus making it possible to determine for the points of intersection the secular changes in the magnetic elements.
After a thorough overhauling in 1919 and the installation of a four-cylinder gasoline engine, made of bronze throughout, to take the place of the producer-gas engine used on earlier cruises, the Carnegie started on her sixth cruise with a crew of twenty-three officers and men on October 9, 1919. A cruise of 61,500 miles was planned in the South Atlantic, Indian and Pacific Oceans to last approximately two years. Unsurveyed regions in the South Atlantic and Indian Ocean were to be covered and the route was planned so as to obtain a large number of observations of the progressive changes that have taken place in the magnetic elements. This is accomplished as stated above by intersecting former routes and obtaining new values of the element at the points of intersection.
In addition to its ocean magnetic surveys the Department of Terrestrial Magnetism also carries on extensive land surveys in all parts of the globe. In 1919 special expeditions were sent out by the Department to observe the total solar eclipse of May 29th at stations distributed over the entire zone of visibility of the eclipse and immediately outside. At Dr. Bauer's station in Liberia the total phase was visible in a cloudless sky for more than six minutes, which is very close to the maximum length of phase that can possibly be observed. Unmistakable evidence was gathered at all stations of an appreciable variation in the earth's magnetic field during a solar eclipse, which variation is the reverse of that causing the daylight portion of the solar diurnal variation of the needle.
In addition to the magnetic survey work on land and sea which is the chief work of the Department of Terrestrial Magnetism, atmospheric-electric observations are carried on continually on land and sea and experiments have been carried on at Langley Field, Va., lately, in the development of methods and instruments for determining the geographical position of airplanes by astronomical observations. There has also been recently formed under this department a Section of Terrestrial Electricity.
The cause of the earth's magnetic field is still one of the greatest unsolved problems of astro-physics. The theory that has been advanced by Schuster that all large rotating masses are magnets as a result of their rotation has received considerable attention from astrophysicists, and attempts have been made to prove this experimentally. It has been found that iron globes spun at high velocities in the laboratory do not exhibit magnetic properties. This may mean simply that the magnetic field is too weak to be detected in the case of a comparatively small iron sphere spun for a limited period under laboratory conditions. It must be remembered that the earth has been rotating rapidly on its axis for millions of years and is, compared to terrestrial objects, an extremely large mass. Yet it has been shown that as a whole our earth is an extremely weak magnet, and that if it were made entirely of steel and magnetized as highly as an ordinary steel-bar magnet, the magnetic forces at its surface would be a thousand times greater than they actually are.
If it is true that all rotating bodies are magnets, then all the heavenly bodies, planets, suns and nebulæ are surrounded by magnetic fields. We know nothing to the contrary. In fact, we know this to be true for the earth and sun, and strongly suspect that it is so in the case of the planets Jupiter and Saturn.
When we understand more about the properties of matter, the nature of magnetism, as well as of gravity, may be revealed to us.